This application claims benefit of Japanese Patent Application No. 2001-211787 filed on Jul. 12, 2001, the contents of which are incorporated by the reference.
The present invention relates to optical modulator drive circuits and, more particularly, to optical modulator drive circuits for driving optical modulator such as laser diodes.
A laser beam emitted from a laser diode is used for such purposes as modulation of optical signal in optical fiber communication and reading of signal from a CD (Compact Disc) and DVD (Digital Versatile Disc). A laser diode drive circuit is used for generating a desired fixed laser beam for such a laser diode.
Prior art examples of such laser diode drive circuit is disclosed in, for instance, a Japanese Patent Laid-Open No. 3-73917 entitled xe2x80x9cOptical Modulator Drive Circuitxe2x80x9d. FIG. 4 shows a typical laser diode drive circuit or modulation circuit. Referring to FIG. 4, a laser diode 110 is serially connected between an output terminal 106 of a semiconductor integrated circuit (i.e., drive IC) 100 and a power supply 121. The drive IC 100 comprises a pair of differentially connected transistors 101 and 102 and a current source 103 connected to the common connection emitter of these transistors 101 and 102.
A modulation signal or like drive input signal is inputted (i.e., applied) between input terminal 104 and 105 connected to the bases of the differential transistors 101 and 102 in the drive IC 100. The collector of the transistor 101 is connected to a power source 122, and the collector of the transistor 102 is connected via the output terminal 106 and the laser diode 110 to the power source 121. The current source 103 has its other terminal connected to a power source 123.
In the modulation circuit or laser diode drive circuit as shown in FIG. 4, by a modulation signal or a drive signal inputted between the input terminals 104 and 106 of the drive IC, the current in the current source 103 is distributed to the collectors of the transistors 101 to 102 for the control of the drive current in the laser diode 110, which is connected to the output terminal 106 as the collector of the transistor 102. Based on the drive current, the laser diode 110 outputs a modulated optical signal.
FIG. 5 shows an optical modulator drive circuit as another prior art example of the laser diode drive circuit. In the optical modulator drive circuit, a load 230 which is constituted by a resistor or an inductor, is connected between the output terminal 206 of the drive IC 200 including a MOS transistor 201 and a power source 221. An optical modulator 210 including a laser diode is connected via an AC coupled capacitor 231 between the output terminal 206 and the power source 222. An input terminal 204 is connected to the gate of transistor 201, which has its source connected to a power source 223 and the drain connected to the output terminal 206 noted above.
The drain current is changed according to a modulation signal inputted to the gate input terminal 204 of the transistor 201 constituting the drive IC 200. The modulation signal which is based on change in the output voltage from the output terminal 206 flows through the AC-coupling capacitor 231 in the optical modulator 210.
In the semiconductor integrated circuit, however, the breakdown voltage is reduced with increasing operation speed. On the other hand, a high voltage is necessary for driving the laser diode. Therefore, the output voltage of the drive IC of such optical modulator may exceed the breakdown voltage of the drive IC and cause rupture of the IC.
The present invention has been made in view of the above prior art, and its object is to provide an optical modulator drive circuit with a low voltage as the output voltage of the driver IC.
According to an aspect of the present invention, there is provided an optical modulator drive circuit for driving an optical modulator by applying a drive signal from a drive output of the optical modulator drive circuit to the optical modulator, wherein a voltage clamp circuit is provided between the drive output and the optical modulator to clamp the voltage at the drive output to be lower than the breakdown voltage of the drive circuit.
According to another aspect of the present invention, there is provided an optical modulator drive circuit for driving an optical modulator by applying a drive signal from a drive output of the optical modulator drive circuit to the optical modulator, wherein a voltage clamp circuit is provided between the drive output and a load connected to a power supply to clamp the voltage at the drive output to be lower than the breakdown voltage of the drive circuit and the optical modulator is connected through a coupling capacitor to the drive circuit.
The voltage clamp circuit includes a transistor with a main current path thereof connected to the output terminal of the drive IC, a constant voltage being applied to a control terminal of the transistor. The voltage clamp circuit includes a uni-polar transistor with a main current path thereof connected to the output terminal of the drive IC, a constant voltage being applied to a control terminal of the transistor. The drive IC includes a pair of differentially connected transistors and a current source connected to the pair transistors. The drive IC is constituted by a single MOS transistor. The drive IC includes a MOS transistor and a current source connected in parallel with the transistor. The load is a resistor or an inductor. The drive circuit is semiconductor integrated circuit.
Other objects and features will be clarified from the following description with reference to attached drawings.